1. Field of the Invention
The present invention relates to a film capacitor having reliable and excellent moisture resistance, which is suitable as an electronic component used in electronic or electric appliances, and a method for manufacturing the same.
2. Description of the Related Art
A film capacitor is required to have improved moisture resistance to improve its performance as an electronic component. Also, studies are directed to make a film capacitor, as well as electronic components in general, smaller and lighter. Regarding the moisture resistance, a film capacitor has conventionally been rated under the condition of 40.degree. C. and 95% relative humidity (RH). Recently, however, the moisture resistance of a film capacitor is required to be rated under more severe conditions, such as those of 60.degree. C. and 95% RH, or 85.degree. C. and 85% RH, or 121.degree. C., 2 atmospheres and 100% RH (a condition of a pressure cooker test).
Among various film capacitors, there is known a metallized film capacitor which is made by forming a metallic layer on a surface of a dielectric base film and then stacking the thus metallized film. Because a metallized film capacitor has very thin metallic layers as electrodes and can be made small, it has found an increasing demand especially as an electronic component for industrial machines used under medium or high voltage. A metallized film capacitor, however, generally has a problem of poor moisture resistance and is far from satisfactory under the above-mentioned moisture conditions.
A metallized film capacitor includes a wound-type capacitor such as shown in FIG. 5, and a stacked-type capacitor such as shown in FIG. 6. Each of the capacitors can be manufactured in the following manner. First, a multilayer structure of either a wound metallized film or stacked metallized films is prepared. Then, a pair of outer electrodes 12 are formed on two side faces of the multilayer structure opposing to each other. For example outer electrodes are formed by the use of a metal spraying method, thereby obtaining a capacitor element. Finally, the capacitor element is either placed in an appropriate casing or covered otherwise, thereby yielding a desired capacitor.
In the manufacture of a stacked-type capacitor, the manufacturing procedure can be described more specifically as follows. First, metallized films having a rectangular shape are stacked to each other, thereby giving a multilayer structure. Second, a pair of outer electrodes are formed on two side faces of the multilayer structure opposing to each other, so as to cover the longer edges of the rectangular metallized films. Then, the thus obtained multilayer structure having a pair of outer electrodes is cleaved along the faces normal to the longer edges of the rectangular metallized films, thereby obtaining capacitor elements of specific sizes. Finally, the facets generated by the above-mentioned cleavages are generally covered by resin layers, thereby yielding desired stacked-type capacitors.
To the inside of the above-mentioned metallized film capacitors as shown in FIGS. 5, 6A and 6B, moisture penetrates easily through the outer electrodes 12, because these outer electrodes 12 have porous structures. The penetration of moisture also occurs easily through the dielectric films 11. In the stacked-type capacitor as shown in FIGS. 6A and 6B, each of facets 14, which is generated by the cleavage during the above-mentioned manufacturing process, is covered by a resin layer 13. The resin layer 13, however, can not completely prevent the penetration of moisture. The moisture that reaches the inside of the capacitor may be condensed, resulting in defects of the capacitor such as a decrease in the electric capacity.
In order to minimize the above-mentioned problem, the capacitor element of a metallized film capacitor has been either placed in a sealed resin casing or provided with a sufficiently thick resin layer as the resin layer 13. The resin layer 13 may be extended so as to cover the entire surface of the capacitor element.
FIG. 7 is a conceptual view showing a partial cross section of a stacked-type capacitor. The capacitor element of the stacked-type capacitor includes a multilayer structure made of metallic layers 15 and dielectric films 16, and outer electrodes 17 electrically connected to the metallic layers 15. A side face of the multilayer structure is covered by a resin layer 13 which is made of urethane resin or epoxy resin. In a conventional capacitor such as shown in FIG. 7, the resin layer 13 cannot completely prevent the penetration of moisture 21 from the outside of the capacitor. Once penetrated into the inside of the capacitor, moisture may be easily condensed according to the change in temperature, thereby giving water particles 22 and 23. These water particles 22 and 23 cause corrosion and elution of the metallic layers 15, resulting in a decrease in the electric capacity. Furthermore, the resin layer 13 which is in contact with the metallic layers 15 tends to be partially removed to give interspaces along the facet (i.e., cleaved surface) of the multilayer structure. These interspaces may collect the water particles 22 and 23, thereby accelerating the corrosion of the metallic layers 15. The facet of the multilayer structure is susceptible to corrosion especially when voltage is applied to the capacitor, because of the electrolytic reaction.
Regarding the size of the capacitor, a microminiature chip metallized film capacitor has been recently developed, which has a contact area of 3.2 mm.times.1.6 mm with regard to a substrate on which the capacitor is to be mounted. In such a microminiature capacitor, a resin layer required to provide the capacitor with moisture resistance occupies a considerable volume of the capacitor, thereby preventing the further miniaturization of the capacitor.
Japanese Laid-Open Patent Publication No. 63-181409 discloses a method for coating a thin resin layer on the surface of a chip film capacitor except for the surface of the outer electrodes of the capacitor and for the surface of the capacitor which is to be in contact with a printed circuit board. Japanese Laid-Open Patent Publication No. 64-77911 discloses a method for forming resin layers on the two facets generated by the cleavage of a chip film capacitor. Japanese Laid-Open Patent Publication Nos. 64-77913, 64-77917 and 64-77920 disclose methods for fixing by heat and pressure a specific sheet material as a coating layer on the surface of a capacitor. Japanese Laid-Open Patent Publication No. 2-43718 discloses a method for applying and curing a resin layer on the facet of a capacitor. According to these methods, the volume of the coating-layer of a chip film capacitor can be made significantly small.
As a metallized film capacitor is further miniaturized, precise operation becomes more difficult for coating a resin sheet thereon or applying a resin solution thereto. With regard to productivity, an advantageous coating method would be, for example, a method of immersing a number of capacitor elements at the same time into a resin solution. This coating method, however, cannot be employed for the manufacture of a chip film capacitor, because the resin layer is required to be formed only on specific faces including the facets of the capacitor, without covering the outer electrodes of the capacitor.